Disposable sewing machine bobbin

ABSTRACT

An improved prewound disposable sewing machine bobbin has a plastic core tube for superior strength and dimensional stability. The ends of the core tube are reduced in wall thickness, preferably by internal beveling, before assembly, thereby providing better crimping and reduced thread breakage during bobbin assembly.

limited States Patent [191 Devine Dec. 11, 1973 DISPOSABLE SEWING MACHINE BOBBIN [75] Inventor: Howard E. Devine, Johnston, R.l.

[73] Assignee: The American Thread Company,

Stamford, Conn.

[22] Filed: Nov. 1, 1971 [21] Appl. No.: 194,240

[52] U.S. Cl. 242/169, 242/1 18.7 [51] Int. Cl B65h 55/00 [58] Field of Search 242/169, 159, 168,

[56] References Cited UNITED STATES PATENTS 2,098,219 11/1937 Baker ..242/169 2,720,179 10/1955 Strocco 242/169 2,957,642 10/1960 Ben0it.... 242/1 18.7 3,471,607 10/1969 Cone 242/169 1 3,501,110 3/1970 Hopgood et al. 242/1 18.7

Primary Examiner-Stanley N. Gilreath Attorney-James H. Callahan et al.

[57] ABSTRACT An improved prewound disposable sewing machine bobbin has a plastic core tube for superior strength and dimensional stability. The ends of the core tube are reduced in wall thickness, preferably by internal beveling, before assembly, thereby providing better crimping and reduced thread breakage during bobbin assembly.

5 Claims, 4 Drawing Figures DISPOSABLE SEWING MACHINE BOBBIN BACKGROUND OF THE INVENTION pensive for one-time use, these disposable bobbins are usually made of paper.

A typical disposable bobbin has a laminated tubular core made from a strip of relatively thin paper, coated with glue, and rolled up on a mandrel. After the core has dried thoroughly, the completed bobbin is assembled by first winding a coil of thread on the core, then placing a stiff, annular manila paper disc over each end of the core against the sides of the coil of thread, and

- finally flaring the ends of the core outward against the faces of the discs to form a unitary riveted assembly. The above assembly steps are usually performed successively on a thread winding machine, a discing machine, and a pressing or riveting machine, respectively.

Although considerably cheaper than its metal counterpart, the paper-cored bobbin suffers from a number of drawbacks. The paper from which the core is made must be water absorbent and be wetted to properly take the glue during the core rolling step. Afterwards, the cores must be carefully dried through a long curing process of several weeks during which the ambient temperature and humidity must be carefully controlled in order to maintain the desired core dimensions.

If the absorbency of the paper is not uniform, the plies will separate in regions where the paper is less absorbent and cause bulges in the tube wall. Or the last ply may come unglued and lift up. Also, if the weather is damp, the cores will absorb moisture and expand. In each case, the cores may jam the winding, discing, or pressing machines during bobbin assembly. In addition, the core ends do not always flare properly during the riveting step, and the discs may detach in subsequent handling operations.

An even greater disadvantage is that bobbins that successfully pass the quality and dimensional tests at the manufacturers plant may swell or lose their discs after delivery to a customer, thus leading to a high rate of returns and to customer dissatisfaction.

SUMMARY OF THE INVENTION The improved sewing machine bobbin of the present invention eliminates the disadvantages of the prior art disposable bobbin by providing a homogeneous core tube with superior dimensional stability. The core is made of a material capable of being cold formed, such as a thermoplastic material, and has a special shape that enables it to be used successfully with conventional bobbin assembly machines.

Instead of being a simple cylindrical tube with constant internal and external diameters as is the prior art paper core, the plastic core tube of the present invention has a region of reduced wall thickness, preferably an interior bevel, at each end. The bevel angle is preferably in the range of about 15 to about 18 with respect to the core axis. Larger or smaller bevel angles may also be used, depending on the relation between factors such as core length before and after riveting and type of thread, whether cotton or synthetic.

The bevelled ends are an important feature of the invention because the plastic tube is less compressible than the prior art paper tube. Unless the wall thickness of the tube ends is reduced there will be excess material resulting from the riveting step that will tend to flow axially inward and consequently displace the center material of the core radially outward. The resulting strain on the threads previously wound on the core can cause serious weakening or even rupture.

Accordingly, it is a primary object of the present invention to provide a disposable sewing machine bobbin having superior dimensional stability over a wide range of temperatures and humidities.

It is another object of the invention to provide a bobbin with a plastic core capable of assembly on conventional paper core bobbin machines.

It is another object of the invention to provide a plastic-cored bobbin competitive in overall cost with conventional paper-cored bobbins. I

Other objects and features of the present invention will be apparent from the following description of the preferred embodiment, in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an end view of a typical plastic bobbin core of the invention prior to assembly.

FIG. 2 is a cross section of the core of FIG. 1 taken along lines 2-2.

FIG. 3 is a cross section ofa bobbin core, wound with thread and having the paper discs in place, mounted on the mandrel of a riveting machine just prior to the riveting step.

FIG. 4 is a view similar to FIG. 3 taken at the instant of maximum compression of the riveting dies.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS. 1 and 2, the improved bobbin core of the invention prior to assembly is in the form of a tube 1 having a cylindrical outer surface 2 and a coaxial cylindrical inner surface 3. At each end of the tube, inner bevelled regions 4 and 5 intersect the cylindrical portion of the inner surface 3 at lines 6 and 7 and the end surfaces 8 and 9 at lines 10 and 11, respectively. The angle that bevelled region 4 or 5 makes with the core tube axis is preferably about 15 to about 18 degrees.

The core tube 1 is made of a plastic material capable of being swaged (i.e., cold formed) and of retaining its swaged shape with minimum spring back after removal from the forming die. Preferred materials for this application are the thermoplastic materials known generically by the term acetal resins because of their high stiffness and resistance to creep and their excellent dimensional stability.

Core tube 1 may be initially formed to size by conventional methods as, for example, by injection molding. After molding the tube is immediately ready for assembly without requiring a prolonged curing period, as is the case for conventional paper cores. The first step in assembly is to wind thread on the core to form a coil of desired length and diameter. This step is preferably performed by a conventional automatic thread winding machine identical to those used for paper bobbin cores.

These thread winding machines have a small spring finger that holds the first few turns of thread against the outer surface of the core until there is enough friction to prevent the thread from slipping as the core revolves. Because the outer surface of the plastic bobbin core as it comes from the injection mold is quite smooth and slippery, it has been found advantageous to roughen this surface to provide a better grip for the thread. A suitable surface can be obtained by barrel tumbling the core tubes with an abrasive for approximately ten minutes.

The coil of thread as wound on the bobbin core is shorter than the core length to leave enough core extending from either end for carrying punched manila paper discs to form the ends of the completed bobbin spool. These punched paper discs have a center hole slightly smaller than the outside diameter of the bobbin core so they will wedge tightly when they are pushed on to the ends of the core, preferably in a conventional discing machine.

After the disc assembling step, the bobbin is ready for the final riveting step. Referring to FIGS. 3 and 4, the core tube 1, prewound with a coil of thread 12 and carrying paper end discs 13 and 14, is placed on a mandrel 15 of a conventional riveting machine (not otherwise shown).

Mandrel 15 extends through a flaring die bushing 16 that is press fitted in a chuck 17. An identical chuck 18 and flaring die bushing 19 are mounted in coaxial opposed relation for reciprocal motion toward chuck l7 and bushing 16, as shown in FIG. 4. As chuck l8 and flaring die 19 move toward chuck l7 and flaring die 16, bevelled surfaces 4 and 5 of the bobbin core tube engage annular grooves 20 and 21 in flaring dies 19 and 16, respectively. Continued axial movement of the chucks towards each other causes the ends of the plastic core tube 1 to curl outwardly in the flaring dies and crimp tightly against end discs 13 and 15. At the same time, the chuck faces compress the coil of thread 12, thereby shortening its length and expanding its diameter to the desired dimensions of the finished thread bobbin assembly. The chucks then move away from each other, mandrel 15 withdraws through bushing 16, and the finished thread bobbin drops into a collection bin.

The importance of bevelling the inner surface at each end of plastic core tube 1 and the ensuing advantages are apparent from FIGS. 3 and 4. First, the bevelled ends fit smoothly and easily into the grooves of the flaring dies, as shown in FIG. 3. Secondly, as mentioned above and shown in FIG. 4, the closing chucks during the riveting step compress the coil of thread axially, thereby expanding it radially. At the same time, the core tube ends curl outwardly, tending to exert additional radial pressure against the threads.

It has been found that if the wall thickness of the plastic core tube is maintained constant throughout its length, a substantial number of completed thread bobbins have broken or weakened threads. Occasionally the entire coil bursts along a radial plane. The principal reason seems to be that there is too much core material, and, as is apparent from FIG. 4, there is no place for it to go but radially outward when the riveting dies are fully closed.

In addition, thicker ends have greater resistance to flaring and tend to transmit the force ofthe closing dies axially inward toward the center portion of the core tube. Since the tube is constrained from moving radially inward by mandrel 15, it must, of necessity, exert pressure radially outward.

Bevelling the ends of the tube thus accomplishes the dual advantages of eliminating some tube material and making it easier to flare the tube ends without transmitting substantial axial crushing forces. At the same time, the flared tube ends curl well out and recurve back to grip the paper end discs tightly. The problem of detached end discs is thus eliminated.

The precise angle and length of bevel to use for a given application depends on the bobbin size and the type of thread. Cotton thread, for example, is more compressible than synthetic thread and can be squeezed more in the riveting process without breaking. As noted above, bevel angles between about 15 and 18 have given excellent results, but smaller or larger angles may work equally well, depending on the relation between the factors involved.

As a result, the improved plastic bobbin core of the invention produces a completed prewound sewing machine bobbin having superior dimensional stability and strength at an overall cost competitive with conventional paper-cored bobbins and capable of assembly on conventional winding, discing, and riveting machinery.

I claim:

1. In a disposable sewing machine bobbin of the type having a tubular core, a coil of thread prewound thereon, and annular discs pressed over the core against the ends of the coil of thread, the ends of the core being swaged to form a smooth unborken riveted surface over the inner margins of the discs, the improvement wherein the core before swaging comprises:

a tube of swageable thermoplastic material, the tube having squared smooth ends, an unbroken cylindrical outer surface from end to end, and a cylindrical inner surface joining an internal bevelled region of reduced wall thickness at each end.

2. The bobbin of claim 1 wherein the angle made by the surfaces of said bevelled regions and the axis of the core is between about 15 and about 18.

3. The bobbin of claim 1 wherein the core is made of an acetal resin plastic.

4. The bobbin of claim 3 wherein the outer surface of the core is roughened sufficiently to hold a few turns of thread from slipping at the start of winding the coil of thread.

5. The bobbin of claim 1 wherein the angle made by the surfaces of said bevelled regions and the axis of the core is equal to or less than about 18. 

1. In a disposable sewing machine bobbin of the type having a tubular core, a coil of thread prewound thereon, and annular discs pressed over the core against the ends of the coil of thread, the ends of the core being swaged to form a smooth unborken riveted surface over the inner margins of the discs, the improvement wherein the core before swaging comprises: a tube of swageable thermoplastic material, the tube having squared smooth ends, an unbroken cylindrical outer surface from end to end, and a cylindrical inner surface joining an internal bevelled region of reduced wall thickness at each end.
 2. The bobbin of claim 1 wherein the angle made by the surfaces of said bevelled regions and the axis of the core is between about 15* and about 18*.
 3. The bobbin of claim 1 wherein the core is made of an acetal resin plastic.
 4. The bobbin of claim 3 wherein the outer surface of the core is roughened sufficiently to hold a few turns of thread from slipping at the start of winding the coil of thread.
 5. The bobbin of claim 1 wherein the angle made by the surfaces of said bevelled regions and the axis of the core is equal to or less than about 18*. 